a. Field of the Invention
The present invention relates to an electrode and element structure for improving characteristics of an energy trapping thickness resonant piezoelectric device such as a piezoelectric resonator used for generating a high-frequency clock and a multi-mode piezoelectric filter used for processing a high-frequency signal, and relates to a producing method thereof.
b. Description of the Related Art
In recent years, a processing speed and a data transfer speed has become remarkably high in an information device such as a computer and its peripheral devices, and accordingly it is required to generate a high-frequency clock. An energy trapping-type vibrator including a quartz vibrator is used for generation of a high-frequency clock, and thus a high-frequency-type vibrator is required. In the case where high frequency stability is required for a change due to an environment temperature and a deterioration with age, an AT cut quartz is generally used as piezoelectric material. The At cut quartz vibrator has excellent temperature stability of ppm order. In the case where the frequency stability is not required, an energy trapping-type vibrator with piezoelectric ceramic is used.
The description will be given as to the energy trapping-type vibrator on reference to FIGS. 10(A) through 10(D). FIG. 10(A) is a top view, FIG. 10(B) is a cross sectional view, and FIG. 10(c) is an amplitude distribution chart of a typical trapped thickness resonant mode. The resonator is composed of a piezoelectric plate 51, exciting electrodes 52 and 53 which are formed on front and rear surfaces of the piezoelectric plate 51 so as to face each other in a narrower extent than the piezoelectric plate. The theory of the energy trapping is described in detail on pages 82 to 89 of "Elastic Wave Electric Technical Handbook" (published by OHM in Nov. 30, 1991, edited by Japan Society for the Promotion of Science: Elastic Wave Element Technique No. 150 Commission). A resonating mode energy which fulfills the condition that the thickness vibration along a thickness vibrating plate surface is propagated by a exciting electrode section 54 and is attenuated by a peripheral non-electrode section 55 is trapped into the exciting electrode section 54. Since the vibration is not propagated through the end surface of the piezoelectric plate 51, the end surface can be easily retained with a package. Since a vibration energy does not leak in the retaining portion, a high Q value can be realized, and influence on the characteristic by the retention is lessened. A difference in the propagation constant between the exciting electrode section 54 and the non-electrode section 55 is shown by the following two effects:
(1) a cut-off frequency decreases due to a mass load such as an electrode; and PA1 (2) a cut-off frequency of the electrode section decreases due to a piezoelectric effect.
In the case of a piezoelectric plate with a small electromechanical coupling factor such as an AT cut quartz thickness shear vibrator, an energy is trapped mainly by the mass load of a provided exciting electrode, and in a piezoelectric ceramic with a large electromechanical coupling factor, the trapping effect such that the piezoelectric effect is overwhelmingly larger is exhibited.
FIG. 10(D) is a frequency chart of a typical resonating mode in the case where a thickness vibration is propagated with above cut-off frequency. A vertical axis represents a frequency, fc represents a cut-off frequency of the non-electrode section, and fc' represents a cut-off frequency of the electrode section. A horizontal axis represents degree of the energy trapping, and the value is represented by a exciting electrode length (L/H) which is standardized by a thickness H of the piezoelectric and a standardized frequency decrement .DELTA.(fc-fc')/fc. Curved lines in the chart represent changes in frequencies of respective resonating modes according to the degree of the energy trapping. S0 and A0 are respectively fundamental resonance of a symmetric mode and anti-symmetric mode, Sn and An (here, n is the order of the thickness resonance, n&gt;1) represents respectively the higher order modes of the symmetric and anti-symmetric modes. In the case of a resonator having symmetric exciting electrode pattern, the anti-symmetric modes can not be excited, because the electromechanical coupling of whole resonating part is completely canceled. In the case of a resonator having no symmetry or a filter, mentioned later, the anti-symmetric modes can be excited.
Here, the conditions of trapping of the respective resonating modes are considered. Since a thickness vibration with a frequency lower than the cut-off frequency of the electrode section and non-electrode section is not propagated through the electrode section and non-electrode section, the trapping does not occur. Moreover, since a thickness vibration with a frequency higher than the cut-off frequency of the electrode section and non-electrode section is propagated through the electrode section and non-electrode section, the trapping does not occur. Therefore, only when the frequency of the thickness vibration is lower than the cut-off frequency of the non-electrode section and higher than the cut-off frequency of the electrode section, the vibration is trapped in the electrode section, and resonance occurs. For example, in the case where the trapping quantity is A in the drawing, S0 is trapped and resonated, but S1 is propagated also through the non-electrode section with a frequency higher than the cut-off frequency of the electrode section and is not trapped, and thus is not resonated. If the trapping quantity is increased to B, not only S0 but also S1 and S2 are trapped and resonated. Namely, if the trapping quantity is A, i.e., a suitable value, the main resonance is only S0, and an excellent vibrator without spurious (unnecessary) resonance can be obtained. Meanwhile, when the trapping quantity is increased by increasing the frequency decrement or by lengthening an electrode length, spurious (unnecessary) resonance occurs. Since the spurious resonance causes a jump of frequencies and unstable operation in a clock generator, the frequency decrement and electrode length should be selected so that the spurious resonance is suppressed.
Next, the energy-trapping-type piezoelectric filter is in great demand with the recent spread of individual mobile communication devices such as portable telephones and pagers. A surface acoustic wave filter and dielectric filter are used in a RF section where a frequency is high and surface acoustic wave filter and quartz filter are used in 1st-IF, and a ceramic filter is used in 2nd-IF, namely, multi-stage filters are used. In these filters, the energy-trapping-type multi-mode piezoelectric filter is a portion of a quartz filter and ceramic filter, and it composes a system with extremely small number of stages. For this reason, these filters require a producing method where the channel selection is varied and the characteristic is stable. Moreover, since a number of portable telephone subscribers has increased rapidly, a number of channels is insufficient, and thus a higher RF frequency is used. Accordingly, an IF filter having a higher frequency is required.
Here, as one example of a conventional energy trapping-type multi-mode piezoelectric filter, a quartz MCF (Monolithic Crystal Filter) is explained on reference to FIG. 11. An AT cut quartz is used as a piezoelectric plate 91, and an input electrode 92a and an output electrode 92b are formed on the front surface of the piezoelectric plate 91, and a common earth electrode 93 is provided to the rear surface of the piezoelectric plate 91. The piezoelectric plate 91 is fixed to pins 95a, 95b and 95c provided on a base 94 of a can package through a conductive paste 96, and the input and output electrodes and the earth electrode are taken out to the outside. Finally, a metallic cap 97 is welded to the base 94 so that the base 94 is sealed.
Similarly to the vibrator, the filter also adopts the theory of the energy trapping, and a thickness vibrating energy is trapped by a mass load into the portion where the electrodes have been formed. The portions of the input electrode 92a and output electrode 92b for respectively different energy trapping vibrators. The two vibrators are positioned at a suitable distance, and when leaked vibration is coupled, the symmetric mode for vibrating input and output sides with the same phase and the anti-symmetric mode for vibrating them with anti-phase (opposite phase) are resonated. When the symmetric and anti-symmetric modes which are propagated from the input side to the output side are controlled, a desired filter characteristic is obtained.
Compared to the vibrator, the filter requires the, more definite electrode design. In the case of the vibrator, the spurious resonance of a higher order mode may be avoided to some degree, but in the case of the filter, it is fundamental that the thickness and size of the electrodes are determined so that the energy is trapped only in the symmetric and anti-symmetric modes which are foundations of the input and output electrodes. If the weight and area of the electrodes are increased, the higher order mode with a high frequency is reflected at the end portions of the electrodes and is resonated, and this causes the spurious resonance. Therefore, the allowable thickness and size of the electrodes are limited, and degree of freedom of the filter design is remarkably restricted.
In the conventional vibrator and filter, gold is used as an electrode material which has stability of a high frequency, a foundation layer such as chrome is used. As the other materials, silver is used to reduce the cost. Further, in the case of a high frequency, metal such as aluminum with small specific gravity is used.
As the low-priced method of forming an electrode thin film pattern on a piezoelectric plate, a metal mask having hole pattern same as the electrode pattern is generally used. In a way which requires higher frequency and the producing method where the more excellent filter characteristic and the stability, higher working accuracy is required, and thus instead of the metal mask, working employing the photo-lithography method is carried out. In such a strict way, it is necessary to align the pattern of the front electrode with that of the rear electrode accurately, thereby increasing the producing cost.
The can package is explained as embodiment, but with the miniaturization of equipments, small-sized vibrator and filter is required, and thus a piezoelectric plate is mounted into a surface mounting package with the piezoelectric plate being laid.
As exemplified above, for designing of the energy trapping piezoelectric devices, how to suppress the level of the spurious resonance is the most important point. In the case where the piezoelectric device is the vibrator, the spurious resonance causes the jump of a vibrating frequency of the vibrator and unstable vibrating state. Moreover, in the case where the piezoelectric device is the filter, since a thickness vibration of a frequency higher than a pass band is easily propagated from the input to output sides, when the vibration is reflected at the end portion of the electrode, the spurious resonance forms an unnecessary pass band with a frequency which satisfies the resonance condition. As mentioned above, it is required that the electrode thickness and length are satisfies the condition so that the spurious resonance does not occur, but it cannot be always realized.
For example, the vibrator and filter with low impedance are required. This is because the vibrator causes stably vibration, and the filter make impedance adjustment with a circuit easy. The impedance can be decreased by increasing the electrode area, but the higher order mode is also resonated, and this causes the spurious resonance. The electrode size can be enlarged without spurious resonance by thinning the electrode thickness, whereas the ultra-thin electrode film has an unstable state such that fine electrode particles are locally coupled. For this reason, the thinning of the electrode thickness is limited. The lower limit of the electrode thickness depends on an electrode material, but a thickness of at least 50 nm is required for obtaining a practically stable electrode film. Moreover, when electrodes with large specific gravity made of gold, silver and the like is replaced by electrodes made of aluminum whose specific gravity is smaller, the mass load can be decreased. However the electrode made of aluminum changes more easily than the electrode made of gold and silver, and thus there arises a problem of long-period reliability. Furthermore, in the case where piezoelectric plates with a large electromechanical coupling factor made of piezoelectric ceramic, lithium tantalate and lithium niobate are used, even if the mass load of the electrodes is zero, a constant energy is trapped by the piezoelectric effect, and thus the upper limit of the electrode size in which the spurious resonance does not occur becomes fairly small. Therefore, the lowing the impedance of the device which ensures no spurious resonance and stable operation is limited.
In the case where a frequency is higher, the wavelength of the thickness vibration becomes short, so it is necessary to relatively reduce the mass load due to the electrodes. Therefore, it is necessary to suppress the spurious resonance by thinning the electrode or by reducing the electrode area, if the electrode cannot be thinned.
In addition, in the conventional energy trapping piezoelectric device, since the taking-out section of the electrode is also mass-loaded, the thickness vibration is propagated through the taking-out section of the electrode and reflected from an unexpected place, thereby causing the spurious resonance. Since it is difficult to analytically design the behavior of the taking-out section, the trial production has been repeated so that the design is determined.